In certain broadcast communications systems, such as, for example, satellite radio, in order to optimize the utilization of a fixed bandwidth, hierarchical modulation (“HM”) can be used to overlay data for new services on top of a legacy transmission, Such a scheme can be used, for example, to offer additional channels or services. For example, in the Sirius XM Radio Inc. (“Sirius”) Satellite Digital Audio Radio Service (“SDARS”), video channels can be sent over existing audio channels via such an overlay modulation scheme, where the video signal is sent in a “Layer 2” or overlay modulation layer. Similarly, such Layer 2 data can be used for other types of content, such as data, traffic, etc.
Additionally, it is well known in the SDARS community that signal reception for polar orbiting satellites is a function of satellite position in the sky. As a satellite approaches its lowest position in the sky, the reception is poorest, and conversely, it is best when the satellite is at it highest position in the sky. This has been a limiting factor in satellite radio broadcasting systems from their outset. Unfortunately, this problem is not easily rectified, which, ultimately, along with many other factors, can limit the overall throughput of such broadcast satellite services.
Sending overlay bits, such as for a video service, for example, compounds this problem, as these bits appear as noise to a legacy receiver (i.e., a standard SDARS receiver not demodulating the overlay layer modulation prior to passing the received signal to a legacy signal demodulator). For example, the Sirius Backseat TV™ video service has been implemented by hierarchically modulating the original (i.e., legacy audio) Sirius QPSK symbol constellation with video data.
As noted, hierarchical modulation, also known as overlay modulation, appears as a noise-like source to legacy receivers, precisely because it further modulates the legacy symbols transmitted such that two symbols having the same legacy bit values generally appear in a different portion of the IQ plane. An additional noise source is un-welcome in any scenario and in the context of satellite radio is simply a consequence to pay if additional bandwidth is to be claimed from an SDARS system.
Fortunately, the additional noise source due to overlay modulation is essentially unnoticeable to legacy receivers for most of the positions of a broadcast satellite in the sky. It is only when a satellite is lowest in the sky (for example, at what are known as Ascending A-node and Descending D-node times, when a satellite's ground track crosses the equator, for a satellite receiver located in North America) that the additional noise-like source due to overlay modulation can cause the most harm to legacy receivers. Such harm is generally manifested as additional mute seconds (FEC coding failure) during the A-node and D-node positions/times of the satellite. Thus, what is needed in the art are systems and methods that can minimize the harmful effects of overlay modulation on legacy receivers as a broadcast satellite approaches points in the sky where reception of its signal is at a minimum.